[TwigBundle] document new autoescape features

This is the documentation for symfony PR 7479 which adds new settings to the TwigBundle that allow to register a custom template escaping guesser

| Q             | A
| ------------- | ---
| Doc fix?      | no
| New docs?     | yes - https://github.com/symfony/symfony/pull/7479
| Applies to    | master
| Fixed tickets | -